In a wire electric discharge machining apparatus, it is general that the larger the machining energy, the faster the machining speed. Therefore, in order to increase the machining speed, the machining energy is increased by increasing the discharge current peak value or the discharge frequency. However, the higher the discharge current peak value or the discharge frequency, the more the risk of wire electrode breakages increases. That is, there exists a limit (wire-breakage limit) to the discharge current peak value and the discharge frequency for continuing discharge operation without wire electrode breakages. In conventional wire electric discharge machining apparatuses, techniques have been developed under the restriction of the wire-breakage limit to increase machining energy as much as possible.
By the way, it is known that the discharge current peak value and the discharge frequency relating to the wire-breakage limit depend on the diameter and material of a wire electrode, the board thickness and the material of a workpiece, and machining conditions including a condition of a machining fluid jet. Among these, the wire electrode and workpiece materials do not vary during a machining operation, however the board thickness of the workpiece and the machining condition may vary. Thus, for efficiently machining without wire electrode breakages, in a conventional technique, the machining condition is controlled according to the board thickness and the machining state which vary during machining (for example, Patent Document 1).